US 20090065855 A1
A semiconductor device is formed on a semiconductor substrate. The device comprises a drain, an epitaxial layer overlaying the drain, and an active region. The active region comprises a body disposed in the epitaxial layer, having a body top surface and a body bottom surface, a source embedded in the body, extending from the body top surface into the body, a gate trench extending into the epitaxial layer, a gate disposed in the gate trench, an active region contact trench extending through the source and at least part of the body into the drain, wherein the active region contact trench is shallower than the body bottom surface, and an active region contact electrode disposed within the active region contact trench.
1. A semiconductor device formed on a semiconductor substrate, comprising:
an epitaxial layer overlaying the drain; and
an active region comprising:
a body disposed in the epitaxial layer, having a body top surface and a body bottom surface;
a source embedded in the body, extending from the body top surface into the body;
a gate trench extending into the epitaxial layer;
a gate disposed in the gate trench;
an active region contact trench extending through the source and at least part of the body into the drain, wherein the active region contact trench is shallower than the body bottom surface; and
an active region contact electrode disposed within the active region contact trench.
2. The semiconductor device of
the device further comprising a termination region comprising:
a second gate trench extending into the epitaxial layer;
a second gate disposed in the gate trench; and
a gate contact trench formed within the second gate.
3. The semiconductor device of
4. The semiconductor device of
5. The semiconductor device of
6. The semiconductor device of
the active region contact trench has a first depth and a second depth;
the first depth is shallower than the second depth; and
a first contact opening corresponding to the first depth is wider than a second contact opening corresponding to the second depth.
7. The semiconductor device of
8. The semiconductor device of
9. The semiconductor device of
10. The semiconductor device of
11. The semiconductor device of
12. The semiconductor device of
13. The semiconductor device of
14. A method of fabricating a semiconductor device, comprising:
forming a gate trench in an epitaxial layer overlaying a semiconductor substrate;
depositing gate material in the gate trench;
forming a body in the epitaxial layer, having a body top surface and a body bottom surface;
forming a source;
forming an active region contact trench that extends through the source and the body into the drain, wherein the active region contact trench is shallower than the body bottom surface; and
disposing a contact electrode within the active region contact trench.
15. The method of
forming a second gate trench that extends into the epitaxial layer;
depositing gate material in the second gate trench; and
forming a gate contact trench within the gate.
16. The method of
17. The method of
18. The method of
19. The method of
20. The method of
21. The method of
22. The method of
23. The method of
24. The method of
25. The method of
forming a hard mask on the substrate prior to forming the gate trench;
removing the hard mask to leave a gate structure that extends above the body top surface.
This application is a continuation in part of co-pending U.S. patent application Ser. No. 11/900,616 entitled POWER MOS DEVICE filed Sep. 11, 2007, which is incorporated herein by reference for all purposes, and which is a continuation of U.S. patent Ser. No. 11/056,346 entitled POWER MOS DEVICE filed Feb. 11, 2005, now U.S. Pat. No. 7,285,822, which is incorporated herein by reference for all purposes.
Power MOS devices are commonly used in electronic circuits. Depending on the application, different device characteristics may be desirable. One example application is a DC-DC converter, which includes a power MOS device as a synchronous rectifier (also referred to as the low side FET) and another power MOS device as a control switch (also referred to as the high side FET). The low side FET typically requires a small on-resistance to achieve good power switch efficiency. The high side FET typically requires a small gate capacitance for fast switching and good performance.
The value of a transistor's on-resistance (Rdson) is typically proportional to the channel length (L) and inversely proportional to the number of active cells per unit area (W). When choosing a value for Rdson, consideration should be given to the tradeoff between performance and breakdown voltage. To reduce the value of Rdson, the channel length can be reduced by using shallower source and body, and the number of cells per unit area can be increased by reducing the cell size. However, the channel length L is typically limited because of the punch-through phenomenon. The number of cells per unit area is limited by manufacturing technology and by the need to make a good contact to both the source and body regions of the cell. As the channel length and the cell density increase, gate capacitance also increases. Lower device capacitance is preferred for reduced switching losses. In some applications such as synchronous rectification, the stored charge and forward drop of the body diode also result in efficiency loss. These factors together tend to limit the performance of DMOS power devices.
It would be desirable if the on-resistance and the gate capacitance of DMOS power devices could be reduced from the levels currently achievable, so that the reliability and power consumption of the power switch could be improved. It would also be useful to develop a practical process that could reliably manufacture the improved DMOS power devices.
Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.
The invention can be implemented in numerous ways, including as a process, an apparatus, a system, a composition of matter, a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. A component such as a processor or a memory described as being configured to perform a task includes both a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. In general, the order of the steps of disclosed processes may be altered within the scope of the invention.
A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.
A metal oxide semiconductor (MOS) device and its fabrication are described. For the purpose of example, N-channel devices with source and drain made of N-type material and body made of P-type material are discussed in detail throughout this specification. The techniques and structures disclosed herein are also applicable to P-channel devices.
Source regions 150 a-d are embedded in body regions 140 a-d, respectively. The source regions extend downward from the top surface of the body into the body itself. While body regions are implanted along side of all gate trenches, source regions are only implanted next to active gate trenches and not gate runner trenches. In the embodiment shown, gates such as 133 have a gate top surface that extends substantially above the top surface of the body where the source is embedded. Such a configuration guarantees the overlap of the gate and the source, allowing the source region to be shallower than the source region of a device with a recessed gate, and increases device efficiency and performance. The amount by which the gate poly top surface extends above the source-body junction may vary for different embodiments. In some embodiments, the gates of the device do not extend above the top surface of the source/body region, rather recess from the top surface of the source/body region.
During operation, the drain region and the body regions together act as a diode, referred to as the body diode. A dielectric material layer 160 is disposed over the gate to insulate the gate from source-body contact. The dielectric material forms insulating regions such as 160 a-c on top of the gates as well as on top of the body and source regions. Appropriate dielectric materials include thermal oxide, low temperature oxide (LTO), boro-phospho-silicate glass (BPSG), etc.
A number of contact trenches 112 a-b are formed between the active gate trenches near the source and body regions. These trenches are referred to as active region contact trenches since the trenches are adjacent to the device's active region that is formed by the source and body regions. For example, contact trench 112 a extends through the source and the body, forming source regions 150 a-b and body regions 140 a-b adjacent to the trench. In contrast, trench 117, which is formed on top of gate runner 131, is not located next to an active region, and therefore is not an active region contact trench. Trench 117 is referred to as a gate contact trench or gate runner trench since a metal layer 172 a connected to the gate signal is deposited within the trench. Gate signal is fed to active gates 133 and 135 through interconnections between trenches 111, 113 and 115 in the third dimension (not shown). Metal layer 172 a is separated from metal layer 172 b, which connects to source and body regions through contact trenches 112 a-b to supply a power source. In the example shown, the active region contact trenches and gate contact trench have approximately the same depth.
Device 100 has active region contact trenches 112 a-b that are shallower than the body. This configuration provides good breakdown characteristics as well as lower resistance and leakage current. Additionally, since the active contact trenches and gate contact trench are formed using a one step process therefore have the same depth, having active contact trenches that are shallower than the body prevents the gate runner such as 131 from being penetrated by the gate contact trench.
In the example shown, the FET channel is formed along the active region gate trench sidewall between the source/body and body/drain junctions. In a device with a short channel region, as the voltage between the source and the drain increases, the depletion region expands and may eventually reach the source junction. This phenomenon, referred to as punch through, limits the extent to which the channel may be shortened. In some embodiments, to prevent punch through, regions such as 170 a-d along the walls of the active region contact trench are heavily doped with P type material to form P+-type regions. The P+-type regions prevent the depletion region from encroaching upon the source region. Thus, these implants are sometimes referred to as anti-punch through implants or punch through prevention implants. In some embodiments, to achieve pronounced anti-punch through effects, the P+ regions are disposed as close as possible to the channel region and/or as close as it is allowed by manufacturing alignment capability and P+ sidewall dopant penetration control. In some embodiments, the misalignment between the trench contact and the trench is minimized by self-aligning the contact, and the trench contact is placed as closely centered between the trenches as possible. These structural enhancements allow the channel to be shortened such that the net charge in the channel per unit area is well below the minimum charge needed to prevent punch through in an ideal unprotected structure. In addition to improving body contact resistance, the anti-punch through implants also makes it possible to construct very shallow trench short-channel devices. In the embodiment shown, contact trenches 112 a-b are shallower than body regions 140 a-d and do not extend all the way through the body regions. The device's on-resistance Rdson as well as the gate capacitance are reduced.
A conductive material is disposed in contact trenches 112 a-b as well as gate trench 117 to form contact electrodes. In the active region, since the punch-through implants are disposed along the sidewalls of the contact trenches but not along the bottoms of the contact trenches, the contact electrodes are in contact with N− drain region 104. Together, the contact electrodes and the drain region form Schottky diodes that are in parallel with the body diode. The Schottky diodes reduce the body diode forward drop and minimize the stored charge, making the MOSFET more efficient. A single metal that is capable of simultaneously forming a Schottky contact to the N− drain and forming good Ohmic contact to the P+ body and N+ source is used to form electrodes 180 a-b. Metals such as titanium (Ti), platinum (Pt), palladium (Pd), tungsten (W) or any other appropriate material may be used. In some embodiments, metal layer 172 is made of aluminum (Al) or made of a Ti/TiN/Al stack.
The leakage current of the Schottky diode is related to the Schottky barrier height. As the barrier height increases, the leakage current decreases, and the forward drop voltage also increases. In the example shown, optional Schottky barrier controlling layers 190 a-b (also known as Shannon layers) are formed below the contact electrode, by implanting thing layers of dopants around the bottoms of active region trenches 112 a-b. The dopants have opposite polarity as the epi layer and are of P type in this example. The Shannon implant is shallow and low dosage; therefore, it is completely depleted regardless of bias. The Schottky barrier controlling layer is used to control the Schottky barrier height, thus allowing for better control over the leakage current and improving the reverse recovery characteristics of the Schottky diode. Details of the formation of the Schottky barrier controlling layer are described below.
Alternative processes may be used. For example, to fabricate devices 106-110 shown in
In some embodiments, to form the Schottky barrier controlling layer, a narrow bandgap material such as SiGe is deposited by chemical vapor deposition (CVD) to form a layer on the top surface of an epitaxial layer. The thickness of narrow bandgap material layer is in the range from 100 Å to 100 Å in some embodiments. For example, a 200 Å silicon rich SiGe layer is used in some embodiments. In some embodiments, the silicon rich SiGe layer comprises 80% Si and 20% Ge. In some embodiments, the narrow bandgap material layer is in-situ doped with N type dopant at a concentration between 2e17 to 2e18/cm3. A low temperature oxide layer is then deposited over the narrow bandgap layer, and patterned to form a hard mask for dry etching trenches into the epitaxial layer. The hard mask protects portions of the narrow bandgap layer underneath during the dry etching process.
The optional modification shown in
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The optional modification shown in
Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.